1. Field of the Invention
This invention relates generally to a modular component connector for joining modular components together and, more particularly, to a method for forming a work hardened modular component connecter for coupling together modular components of a knee, hip or shoulder prosthesis for use in orthopedic surgery.
2. Discussion of the Related Art
Various modular prosthetic devices are currently known and utilized during orthopedic surgeries. These modular prosthetic devices include modular knee, hip and shoulder prosthesis. Since the modular prostheses consist of multiple components, various coupling mechanisms are used to secure or couple the modular components together.
One of the primary and most common coupling mechanisms employed consists of a Morse type taper 10, shown in FIG. 1. The Morse type taper 10 is generally known as a "self-holding" or "self-locking" taper because the angle of the conical taper is generally 2.degree. to 4.degree. which creates considerable friction to lock a male portion 12 into a female portion 14.
However, because of tolerance effects during manufacturing of the Morse type taper 10, it is very difficult to manufacture the male portion 12 and the female portion 14, so that the male portion 12 axially engages along the entire female portion 14. Accordingly, in some instances the male portion 12 may only contact the female portion 14 at one contact point at an unknown location within the Morse type taper 10, as shown by reference numeral 16. This single contact point 16 occurs because in some instances the conical angle of the male portion 12 does not mate with the conical angle of the female portion 14. This condition may in turn allow for micro-motion to occur between the components resulting in fretting at the contact point 16. Such fretting may eventually lead to stress fractures along the loaded contact point 16 that could result in catastrophic failure of the connector.
Moreover, should the male portion 12 axially engage along the entire female portion 14, the Morse type taper 10 also poses other difficulties. For example, where the male portion 12 exits or contacts the female portion 14 at reference point 17, there is generally a stress concentration at this exit point 17. This stress concentration point runs about the circumference of the male portion 12 and is one the areas most likely to fail over time due to the stress concentration along this area.
Other taper connections, such as those disclosed in U.S. Pat. No. 4,917,530 to Englehardt et al. are designed to preclude contact between the male tapered member and the peripheral edge defining the mouth of the bore in the female member. However, Englehardt et al. does not address the problem of having only one contact point between the male and female portions at a location that may vary within the bore, thereby permitting micro motion. Rather, Englehardt et al. assumes axial contact, that is, full face to face contact of the adjacent tapered surfaces of the male portion and female bore and is only concerned with impingement at the mouth of the female bore. Therefore, Englehardt et al does not reduce micro motion caused by practical tolerance effects or the potential resulting stress fractures associated therewith. Nor does Englehardt et al. suggest any ways to increase strength at the stress concentration point 17, other than to reduce contact between the male member and the peripheral edge of the female member.
Other connection mechanisms have also utilized ion implantation along the connection members to provide an ion layer in these areas. However, ion implantation does not appear to provide any appreciable increase in strength because the surface thickness of the implanted ion layer is only generally a few microns thick. Alternatively, mechanical hardening techniques have not even been considered, nor utilized because of the tight tolerance effects of such connections and the inability to perform such mechanical hardening with such tight tolerance effects.
What is needed then is a modular component connector for a modular knee, hip or shoulder prosthesis or any other type prosthesis which does not suffer from the above-mentioned disadvantages. This in turn, will provide a modular component connector that substantially reduces or eliminates micro-motion, reduces the possibility of stress fractures, provides positive contact along at least two predictably located separated contact points for increased stability, provides a mechanically hardened surface at least along one contact point or stress concentration region to provide a thick surface hardening and a stronger connection, and provides a cost effective and easily manufacturable modular component connector. It is, therefore, an object of the present invention to provide a method for forming a work hardened modular component connector for use with modular prosthetic devices.